1. Field of the Invention
The present invention relates to an optical waveform measurement apparatus and an optical waveform measurement method, and particularly relates to an optical waveform measurement apparatus and an optical waveform measurement method for measuring waveform of an optical signal with a high bit rate.
2. Description of the Related Art
A function to observe optical pulse waveforms is necessary for evaluating the quality of the optical signal in receivers of optical communication systems, and it can be applied for the purpose of monitoring the signal quality in optical nodes or optical repeaters in optical communication systems. Meanwhile, along with increase in the capacity for optical fiber communications, the bit rate of the optical signal has reached 40 Gb/s. In addition, as the next generation technology, the research and development of a system transmitting an optical signal at 160 Gb/s or higher is in progress. In order to support such research and development, an optical waveform measurement apparatus and an optical waveform measurement method allowing highly precise observation of the waveform of an optical signal are required. It should be noted that the function to measure or to observe optical pulse waveforms could be used for evaluating a variety of optical components.
The conventional optical waveform measurement apparatus converts a measured optical signal (optical pulse) into an electrical signal using an optoelectronic converter 501, as in an example of FIG. 1. A sampling pulse generation circuit 503 generates a sampling pulse in accordance with a trigger signal generated by a trigger circuit 502. At that time, the sampling pulse is not an optical signal but an electrical signal. A sampling circuit 504 conducts sampling of the electrical signal output from the optoelectronic converter 501 with the sampling pulse. A waveform display device 505 displays the sampling result. In other words, the waveform of an optical signal is observed after the optical signal is converted into an electrical signal.
At that time, in order to observe the waveform of the optical signal with high accuracy, all frequency components constituting the optical signal have to be received. For that reason, the waveform measurement apparatus needs to have a band sufficiently wider than the band corresponding to the bit rate of the optical signal. Here, the band observed by the waveform measurement apparatus shown in FIG. 1 is limited by a device having a narrowest band among the optoelectronic converter, the trigger circuit, the sampling circuit and the waveform display device. However, since the operation band of an electrical circuit is around 50 GHz, highly accurate observation of the optical signal with 40 Gb/s by a waveform measurement apparatus shown in FIG. 1 is considered to be difficult.
For a technology to solve the issue of band limitation, an optical sampling technique, which conducts sampling of optical signals without modification, is known widely. In the following, the optical sampling technique is explained with reference to FIG. 2A and FIG. 2B. Note that FIG. 2B is a schematic drawing of an operation of a waveform measurement apparatus shown in FIG. 2A. Assume that the measured light carries an optical pulse signal with a repetition frequency f0.
An optical sampling gate 511 comprises a nonlinear optical medium, and generates an intensity correlation optical signal of the measured light and the optical sampling pulse. In this description, the intensity correlation optical signal refers to an optical signal generated by the measured light and the optical sampling pulse being overlapped in a time domain.
A sampling frequency signal generator 512 extracts a clock signal (frequency: f0) from the measured light, and generates a sampling frequency signal (frequency: f0+Δf) using the clock signal. A short pulse optical source 513 is driven by the sampling frequency signal. By so doing, an optical sampling pulse is generated. When both of the measured light and the optical sampling pulse are input to the optical sampling date 511, a nonlinear effect occurs. Then, an optical signal with intensity correlation of the measured light and the optical sampling pulse (i.e. the intensity correlation optical signal) is obtained.
The intensity correlation optical signal output from the optical sampling gate 511, after being converted into an electrical signal by the optoelectronic converter 514, is input to a vertical axis signal port of the waveform display device 515. To a horizontal axis input port of the waveform display device 515, a frequency shift signal (frequency: Δf) is input. By so doing, the waveform display device 515 can display the waveform of the measured light. Here, the frequency Δf is substantially low, compared with the frequency f0. Therefore, the issue of the band limitation should be solved.
However, the nonlinear effect, which occurs in the nonlinear optical medium of the optical sampling gate 511, depends on polarization states of the incident measured light and optical sampling pulse. Consequently, accurate observation of the waveform of an optical signal with an arbitrary polarization state is difficult by the optical waveform measurement apparatus shown in FIG. 2A.
Non-patent Document 1 describes an optical waveform measurement apparatus, which solves the above issue of the polarization dependency. The apparatus described in Non-patent Document 1 employs a polarization diversity scheme as shown in FIG. 3. That is, an optical signal is separated into a TE polarization component and a TM polarization component by using a polarization beam splitter (PBS) 521. The TE polarization component and the TM polarization component are input to KTP crystals 522 and 523, respectively, which are provided as a nonlinear optical medium. A pair of the optical signals output from the KTP crystals 522 and 523 is guided to a photo detector element 524. Note that the apparatus comprises transmitters 530 and 531, an optical source 532 for generating an optical sampling pulse, a mode locked fiber laser (MLFL) 533 for generating the measured light, half wave plates (HWP) 534 and 535, mirrors 536-538, an A/D converter 539, and a computer 540.
Non-patent document 2 describes another optical waveform measurement apparatus, which also solves the above issue of the polarization dependency. The apparatus described in Non-patent document 2 comprises polarization controllers for controlling each of the polarization states of the measured light and the optical sampling pulse, as shown in FIG. 4. That is, a polarization controller 541 controls the polarization state of the measured light, and a polarization controller 542 controls the polarization state of the optical sampling pulse. A directional coupler 543 combines the measured light and the optical sampling pulse, each having controlled polarization states, and guides them to an optical fiber 544. The optical fiber 544 is a nonlinear optical medium, and causes a nonlinear effect. A polarizer 545 extracts a prescribed polarization component from the optical output of the optical fiber 544. A wavelength filter 546 extracts a wavelength component of the measured light.
The polarization direction of the measured light is controlled so as to be orthogonal to the polarization main axis of the polarizer 545. The polarization direction of the optical sampling pulse is controlled so that the polarization direction of the optical sampling pulse differs from that of the measured light by 40°-50° (preferably 45°). In such a configuration, if the optical pulse of the measured light overlaps the optical sampling pulse in a time domain, the optical pulse of the measured light is output via the polarizer 545, and if the optical pulse of the measured light does not overlap the optical sampling pulse in the time domain, the optical pulse of the measured light is blocked by the polarizer 545.
It should be noted that Patent Documents 1 and 2, and Non-patent Document 3 are known as technologies relating to the above optical waveform measurement.
[Patent Document 1]
Japanese Patent Application Publication No. 7-98464
[Patent Document 2]
Japanese Patent No. 3494661
[Non-Patent Document 1]
N. Yamada et al., “Polarization-insensitive optical sampling system using two KTP crystals,” IEEE Photonics technology letters, vol. 16, No. 1, pp. 215-217, 2004
[Non-Patent Document 2]
S. Watanabe et al., “novel Fiber Kerr-Switch with Parametric Gain: Demonstration of Optical Demultiplexing and Sampling up to 640 Gb/s,” Postdeadline session, Th4.1.6, 30th European Conference on Optical Communication, Sep. 5-9, 2004 Stockholm, Sweden
[Non-Patent Document 3]
S. Watanabe et al., “Ultrafast All-Optical 3R-Regeneration,” IEICE Trans. Election, vol. E87-C, No. 7, July 2004
The configuration described in Non-Patent Document 1 requires two costly KTP crystals as the nonlinear optical medium. For that reason, cost reduction of the optical waveform measurement apparatus is difficult to achieve.
The configuration described in Non-Patent Document 2 requires setting of the polarization states of the measured light and the optical sampling pulse before operating the optical waveform measurement apparatus. For that reason, if polarization change occurs in the optical fiber during the operation, accurate measurement of the optical waveform is difficult afterwards.